1. Field of the Invention
This invention relates to a method of and apparatus for reducing power consumption of portable radio communication systems, such as mobile telephones, in standby mode to increase the time between necessary battery charges and recharges.
2. Discussion of Related Art
Cellular mobile telephone systems comprise a network of base stations, each covering a particular geographical area or cell, that communicate with a plurality of mobile or hand-portable phones (hereinafter "mobile phones"). When a mobile phone is neither originating nor receiving a call it is in a "standby mode." Although essentially inactive in the standby mode, the mobile phone must listen to the nearest base station to determine if it is being called.
In the American Mobile Phone System (AMPS) EIA-553 cellular standard, the base stations transmit 40-bit word structures to the mobile phones in the standby mode. The word bit structures (or simply "words" or "control words") identify a called mobile phone by including its telephone number, also known as the Mobile Identification Number (MIN), in the message. Each word bit structure also contains a Cyclic Redundancy Check (CRC) code, the value of which depends on the data bits in the word bit structure. This CRC code can be used to verify the correct decoding and to correct single bit errors in the word bit structure. As illustrated in FIG. 1, each 40-bit work bit structure consists of 28 data bits and 12 CRC bits (e.g., Bose-Chaudhuri-Hocquenghem code bits, or commonly known as BCH code bits).
A message word structure (or simply "message") used for transmitting the above-described word bit structure is shown in FIG. 2. By way of overview, the message word structure includes preamble bit-blocks labeled "D" and "S." After the preamble, the above-described word bit structure is repeated a plurality of times. That is, the message word structure conveys five repeats A1, A2, . . . A5 of a first 40-bit word "A" and five repeats B1, B2 . . . B5 of a second 40-bit word "B." The A and B words belong to independent messages intended for mobile phones having odd and even MINs, respectively.
FIG. 3 shows the beginning portion of the message word structure of FIG. 2 in greater detail. The "dotting sequence" D is a 10-bit block of alternating 1's and 0's intended to provide the receiver with symbol resynchronization opportunities. Symbols are transmitted using Manchester code, in which a "1" is represented by a signal upswing followed by a signal downswing and a "0" is reproduced by a downswing followed by an upswing. In the EIA-553 cellular standard, the bit rate is 10 kb/s. The alternating 1's and 0's after Manchester coding then appear as a 5 KHz tone. The Manchester coded bits are transmitted using frequency modulation of a radio carrier. Preceding the 10-bit dotting sequence is a single bit busy/idle flag, giving the total 11 bits labeled "D" in FIGS. 2 and 3. Following the dotting sequence D is an 11-bit sync word also preceded by a busy/idle flag, making the 12 bits labeled "S" in FIGS. 2 and 3. Further, in each repeat word, four extra busy/idle bits are inserted, making 44-bit blocks, as shown in FIG. 3. The total number of bits in a calling channel cycle is thus 1+10+1+11+2.times.5.times.(40+4)=463 bits.
The MIN is 34 bits long. Thus, it takes two calling messages to uniquely identify the phone. The first 24 bits of its MIN are sent in a first cycle (e.g., a fist message word structure) and the remaining 10 bits of the MIN are sent in a second cycle (e.g., a second message word structure), as shown in FIG. 4. The first word of the multiple word message contains a flag to indicate that continuation words are to be expected. Continuation words have a continuation flag set. The continuation flag appears in either the A or B word depending on whether the mobile has an odd or even MIN. A call can in some cases consist of more than two words and so it is necessary to look at the third word to determine if it is a continuation of an already begun message, or of a new message. Other possible message types are dummy messages, or filler, which may be a single word that can be ignored, and a one- or multiple-word broadcast or "overhead" message that is to be processed by all mobile phones.
The above discussion is also relevant to the NMT standard used in Scandinavia and the ETACS standard used in the U.K., as well as other related standards not mentioned above.
In use, the above-described transmission of multiple "A" and "B" words provides redundancy against distortion in the radio channel, such as fades and impulsive noise. For instance, the mobile phone can collect the repeated messages and then use 3/5 majority voting to decode the message. This, in conjunction with the BCH processing, provides an effective level of error detection and error correction.
The above-described protocol presents a number of challenges. Notably, in the standby mode, the mobile phone must operate its receiver section to determine whether it is being called. Needless to say, this monitoring function requires energy which runs down the mobile phone's battery. This may especially present a concern in smaller mobile phones that use small batteries. Some means of conserving the battery supply is therefore desirable.
One way to reduce power consumption in the standby mode is described in European Patent No. EP 0 473 465 to Harte. Harte proposes a phone that decodes each message repeat independently as it is received, carries out a check for correct decoding using the CRC, and if correct decoding is indicated by the CRC check, checks whether the MIN of the receiving mobile phone is contained in the received word. If the MIN is not in the received word, as is the case for the vast majority of messages, then the mobile phone powers down until the next set of five message repeats is due. Thus, according to the European patent, the mobile phone can enter a reduced power mode for the remaining four message repeats whenever the first of the repeats is deemed not to contain the mobile's MIN, representing a potential savings of at least 80% of the battery energy consumed in the standby mode. Additionally, when the mobile phone has already identified from the first of the two control words of a message that the MIN does not match that of the receiving mobile phone, the mobile phone does not need to process the second word, and can power down for all five repeats of the second word. This results in a potential power savings of 90% in the standby mode.
In this technique, a mobile phone will not receive all of the five repeated words if the mobile phone determines that the first word is not addressed to the mobile phone. Accordingly, this technique is not optimally designed to make use of the majority voting of the U.S. EIA-553 standard. A mobile phone using this technique may therefore have reduced performance when confronted with fades and other degradation in the radio channel.
U.S. Pat. No. 5,568,513 to Croft et al. ("the '513 patent") improves upon the Harte technique. The '513 patent is hereby incorporated by reference in its entirety. The '513 patent discloses a technique in which, upon detection of a message, the mobile phone resets a number of accumulators corresponding to the number of data bits and check bits in a word of the message. Upon receiving a first of the repeated words, the mobile phone adds the values of each bit to a corresponding one of the accumulators. The accumulator contents are then processed to determine if values corresponding to the data bits in the accumulators are consistent with values corresponding to the check bits. A check or no-check indication is generated in accordance with this check bit processing. If the no-check indication is generated, another repeat of the word is received and its bit values are added to corresponding accumulators. The technique repeats the above-described processing until all word repeats have been accumulated or until the check signal is generated.
Upon generation of a check indication, the technique powers down parts of the mobile phone for the remainder of the repeats of the word and then further processes the accumulator values to determine if part of the word matches a corresponding part of the portable station's identification number. A match or no-match indication is generated on the basis of the processing. Upon generation of a no-match indication, the technique holds parts of the mobile phone powered down for the duration of transmission of the rest of the words in the same message. In this manner, the '513 patent receives the benefits of reduced power consumption, yet does not sacrifice the also important feature of majority voting to reduce the effects of fades.
While the above-described techniques have several advantages, there remains room for improvement in this field to further reduce the consumption of power of a mobile phone in the standby mode. In particular, it would be desirable to further reduce the time during which the receiver of the portable phone is active.